The present invention relates to the processing of flat work pieces such as semiconductor wafers. More particularly, megasonic energy is applied to a thin layer of fluid directed to a wafer surface through an opening in a transducer faceplate, to clean or etch the wafer surface.
Semiconductor devices are typically fabricated on a substrate in the form of a circular wafer of semiconductor material. Electronic devices and circuitry are fabricated on the semiconductor wafer using one or more available techniques, such as selective etching, photolithography, and vapor phase deposition.
During the fabrication of devices and circuitry on the semiconductor wafers, particulate matter accumulates on the wafer and migrates to where devices and circuitry are being fabricated. Particulate matter that remains on the wafer will cause defects in the devices and circuitry being fabricated on the wafer. Those defects can result in the production of defective electronic devices, reduce the number of functional units per wafer, and increase the cost of wafer production per unit. With the rapid technological advances in semiconductor production, electronic device geometries continue to diminish, and defects in semiconductor wafers, accordingly, have become more critical.
The established method to solve the particulate matter accumulation problem is to immerse the wafers in a fluid bath. The particles attached to the surface of the wafers are small and difficult to remove because they are within the boundary layer of the fluid bath. Accordingly, acoustic energy is added to the fluid bath to aid in breaking the particles loose from the wafer surface. The acoustic energy creates turbulence in the fluid, which effectively reduces the fluid boundary layer thickness. To avoid undesirable effects of cavitation within the fluid, sonic energies with frequencies above 0.5 megahertz (MHz) are used. The term xe2x80x9cmegasonicxe2x80x9d is used to indicate that sonic energy is in the range of 0.5 to 2 MHz. The megasonic energy is commonly applied to the bottom of the tank.
However, fluid baths have seen limited success. To permit bulk handling for cost reasons, multiple wafers are placed in a carrier, or cassette, which is then placed in the fluid bath. With this approach, each wafer is exposed to a different level of sonic energy, often resulting in non-uniform cleaning of the wafers.
The drawbacks with the fluid baths and other developments in sonic wafer cleaners have led to cleaning devices that dispense cleaning fluid onto a single wafer, rotated on a spindle, with a focused sonic energy source, or transducer, located over the wafer to apply ultrasonic energy to the fluid. Cleaning systems of this type have been described, for example, in U.S. Pat. No. 4,064,885 (Dussault et al.), U.S. Pat. No. 4,401,131 (Lawson), and U.S. Pat. No. 4,501,285 (Irwin et al.), U.S. Pat. No. 5,368,054 (Koretsky et al.) and in Japanese patents 61-16528 and 4-213827.
However, that approach also has several drawbacks. First, the fluid used for cleaning is typically dispensed from a pump located independent from the transducer, and flows into a space between the wafer and the sonic source. Uniformity is improved, but due to the necessarily small active energy spot (limited by the size of the transducer) and the fairly high fluid flow requirements, cleaning times are long and consumption of cleaning solution is high.
Second, devices of that type do not provide an efficient way of controlling the energy density of the transducer. The rapid increase in the demand for a variety of electronic circuits on wafers, and for a variety of applications, has led to a concomitant demand for sonic cleaning devices that are able to remove particulate matter from wafers of various materials, with various devices and circuitry. To accommodate that variety, the cleaning device must be able to efficiently adjust the energy applied by the transducer to optimize its energy density for a particular type of wafer being cleaned or etched.
Finally, the transducer can be adversely affected by the cleaning chemicals used. Thus, the chemicals used and their concentrations in the fluid are typically very limited. At the same time, ultrasonic cleaning devices must be able to accommodate higher concentrations of chemicals to efficiently clean the wafers.
The present invention remedies the above disadvantages through, e.g., an improved distribution of the cleaning fluid, an improved transducer assembly, and a system for optimization of the sonic energy density.
The present invention provides a device and a method for treating wafers comprising, for example, an improved delivery of cleaning fluid, an improved transducer lens, and a system for optimization of the transducer energy density. The present invention thoroughly cleans or etches semiconductor material at a desired rate for economical processing thereof.
The present invention applies megasonic energy to clean the wafers by dislodging small particles from the wafer surface, without the use of a fluid tank. A preferred embodiment of the present invention provides an apparatus for treating, e.g., cleaning or etching, wafers, wherein the apparatus further comprises: a wafer support for supporting and rotating a wafer to be treated; a fluid supply port for directing a layer of fluid to the surface of the supported wafer; and an electromechanical transducer assembly for converting electrical signals into mechanical vibrations of a pre-selected megasonic frequency and wavelength and applying the vibrations to the surface of the supported wafer through the layer of fluid. The transducer assembly further comprises (i) a sealed lens having an interior which is bounded by a faceplate and a wall extending upward from the periphery of the faceplate, and a plurality of exterior surfaces, wherein at least one exterior surface of the faceplate portion of the lens comprises a planar fluid contact surface, and (ii) an electromechanical transducer which is located in the interior of the lens and placed in vibration transmitting contact with the faceplate, such that the vibrations are transmitted to the fluid contact surface. The faceplate comprises a material, which is inert with respect to the fluid and which has a thickness that is a multiple of the wavelength of the mechanical vibrations. The fluid supply port is located through a portion of the faceplate.
In another preferred embodiment, a method is provided for treating a wafer comprising: supporting and rotating a wafer to be treated on a wafer support, and directing a layer of fluid to the surface of the supported wafer through a fluid supply port, wherein the thickness of the layer of fluid on the supported wafer is a multiple of xc2xd wavelength of the mechanical vibrations in the fluid. This is followed by sending an electrical signal to an electromechanical transducer assembly, wherein the electrical signal is converted into mechanical vibrations of a pre-selected frequency and wavelength and applying the vibrations to the surface of the supported wafer through the layer of fluid, wherein the transducer assembly comprises (i) a sealed lens having an interior which is bounded by a faceplate and a wall extending upward from the periphery of the faceplate, and a plurality of exterior surfaces, wherein at least one exterior surface of the faceplate portion of the lens comprises a planar fluid contact surface, and (ii) an electromechanical transducer which is located in the interior of the lens and placed in vibration transmitting contact with the faceplate, such that the vibrations are transmitted to the fluid contact surface, wherein the faceplate comprises a material, which is inert with respect to the fluid and which has a thickness that is a multiple of the wavelength of the mechanical vibrations, and wherein the fluid supply port is located in a portion of the faceplate. Finally, the fluid layer in contact with the fluid contact surface is excited to effect the desired treatment on the wafer.
In addition, in the preferred embodiments of the invention, the lens material is a non-metal material preferably quartz or sapphire, wherein the thickness of the faceplate is equal to one and one half wavelengths of the mechanical vibration in the material.
As provided in the preferred embodiment of the invention, the electromechanical transducer assembly is supported from above by a cantilevered arm, wherein the arm moves the transducer assembly radially over a wafer on the wafer support to cover the entire wafer surface. Moreover, in this embodiment, the arm further includes height adjustment means, whereby adjusting the height of the fluid contact surface above the wafer controls thickness of the fluid layer on the wafer by multiples of one-half the wavelength of the mechanical vibrations in the fluid.
The fluid supply port in the preferred embodiment is located in the center of the transducer assembly and through the center of the faceplate.
The invention is applicable to cleaning and etching wafers in semiconductor fabrication operations, as well as to other operations in which megasonic energy is useful in treating other objects.
Additional objects, advantages and novel features of the invention will be set forth in part in the description and figures which follow, and in part will become apparent to those skilled in the art on examination of the following, or may be learned by practice of the invention.